The present invention relates to a safety device which is particularly usable for mechanisms for lifting a back of a chair or armchair.
Mechanisms for lifting a back, of the kind disclosed in Italian Utility Model application TV95U000016 dated Mar. 21, 1995, are currently known which comprise a blade-like element which is slidingly associated with a supporting plate for a back which protrudes to the rear of, and above, a seat.
The blade-shaped element has a first recess whereat a rack is associated; a complementarily toothed ratchet element interacts with said rack, can move only transversely to the rack and is forced toward it by a suitable spring.
This conventional solution, which also includes suitable means associated with the rack for temporarily uncoupling the ratchet with respect to said rack, allows to vertically adjust the back with respect to the seat but entails drawbacks: first of all, vertical adjustment of the back can be achieved only starting from the condition in which the back is at the lower stroke limit, and this condition occurs only if the back has been placed first at the upper stroke limit.
Only starting from this position it is in fact possible to quickly lower the back to the lower stroke limit; this occurs by virtue of the stable locking of the ratchet in a condition in which it does not interact with the rack. Moreover, the conventional device does not have antipanic or antishock characteristics, since if the ratchet is at the lower stroke limit, for example when the chair is moved by holding it by its back, the back may rise suddenly to its upper stroke limit and then suddenly fall again.